function throttle(fn, delay, atleast) {
    var timeout = null,
        startTime = new Date();
    return function() {
        var curTime = new Date();
        clearTimeout(timeout);
        if(curTime - startTime >= atleast) {
            fn();
            startTime = curTime;
        }else {
            timeout = setTimeout(fn, delay);
        }
    }
};
function lazyload() {
    var images = document.getElementsByClassName('.fwr_page_bg_image');
    var len  = images.length;
    var n  = 0;      //存储图片加载到的位置，避免每次都从第一张图片开始遍历
    return function() {
        var seeHeight = document.documentElement.clientHeight;
        var scrollTop = document.documentElement.scrollTop || document.body.scrollTop;
        for(var i = n; i < len; i++) {
            if(images[i].offsetTop < seeHeight + scrollTop) {
                //if(images[i].getAttribute('src') === '../../img/images/Bjlogo.png') {
                    images[i].src = images[i].getAttribute('data-src');
                //}
                n = n + 1;
            }
        }
    }
};


var loadImages = lazyload();
loadImages();          //初始化首页的页面图片
window.addEventListener('scroll', throttle(loadImages, 500, 1000), false);